Production facilities for making asphalt concrete to be used as a paving composition are well-known. Feed materials for these facilities include aggregate materials and asphalt cement. The aggregate materials may be provided in the form of virgin aggregate materials, and/or RAP, and/or RAS. If RAP and/or RAS are included in the feed materials, these components will also provide an additional source of asphalt cement.
Some conventional asphalt concrete production plants employ a rotating dryer drum in which virgin aggregate materials and/or RAP and/or RAS are introduced. A burner is located at one end of the drum and the input feed materials are moved along the drum through the heated gases generated by the burner in either parallel flow or counter-current flow to an outlet. Such a dryer drum is sometimes referred to as a direct-fired dryer or a direct dryer, because the aggregate materials are moved into direct contact with the flames and combustion gases produced by the burner. A separate mixer, such as a rotating drum mixer or a pugmill, is employed to mix the heated and dried aggregate materials with liquid asphalt cement.
Another type of asphalt concrete production plant employs a dryer/mixer that dries and heats the aggregate material and also mixes it with asphalt cement. One such type of dryer/mixer is the DOUBLE BARREL® brand dryer/mixer that is sold by Astec, Inc. of Chattanooga, Tenn. This dryer/mixer includes a generally cylindrical fixed outer drum and a heating chamber comprised of a generally cylindrical inner drum that is adapted to rotate with respect to the outer drum. A burner at one end of the inner drum heats aggregate material by direct exposure to the flames and hot gases generated, and the heated aggregate material is discharged from the inner drum into the outer drum where it is mixed with asphalt cement and/or with RAP and/or RAS. If substantial quantities of RAP and/or RAS are introduced into the DOUBLE BARREL® brand dryer/mixer, a separate mixer such as a pugmill or mixing drum may be employed to incorporate additional asphalt cement into the mixture.
Other asphalt concrete production plants may employ indirect dryers and/or indirect pre-dryers which are adapted to heat and/or dry aggregate material without such material coming into direct contact with a burner flame or with the heated gases generated by a burner. Some such indirect dryers may include a rotating drum that is heated by a system comprising a plurality of thermal-fluid tubes that extend along the interior of the drum and are adapted to circulate a heated fluid along the inner walls of the drum as aggregate material to be heated is passed through the drum.
Whether a dryer/mixer or a dryer with a separate mixer is employed in the production of asphalt concrete, dust is generated as the aggregate materials that are included in the asphalt concrete mix are tumbled through the exhaust gases in the dryer/mixer or dryer. This dust is typically carried upwardly by the hot gases of combustion or other exhaust gases within the dryer/mixer or dryer. Because of particulate emission regulations, it is unacceptable to exhaust the dust-laden gases to the atmosphere. Furthermore, depending on the speed of rotation and the temperature at which the dryer/mixer or dryer is operated, the quantity of dust may represent a significant portion of the fine aggregate material needed in the particular asphalt concrete mix. Therefore, dust collection or recovery systems are known for removal of the dust from the gas stream before further processing of the exhaust gases and/or exhaustion of the same to the atmosphere. The dust which is collected in the dust recovery system may then be introduced to the dryer or mixing chamber for inclusion in the asphalt concrete mixture.
It is known to provide a dust recovery system which includes an inertially driven primary collector such as a cyclone and a filter-based secondary collector such as a baghouse. In such a system, the primary collector will operate to remove the larger sized particles (i.e. larger than about 150 microns) and the secondary collector will remove the remaining particles.
It is often economically advantageous to use a high proportion of RAP and/or RAS in the asphalt mixture. However, it is generally the case that the greater the proportion of RAP and/or RAS in the asphalt mixture, the lower the rate of heat transfer in the dryer or dryer/mixer and the higher the temperature of the dust-laden gas stream from the dryer or dryer/mixer. This dust-laden exhaust gas stream from the dryer/mixer or dryer may be as hot as 350° F. or higher. Exhaust gases at such temperatures may damage the filter media in a filter-based secondary collector such as a baghouse. Consequently, it would be advantageous to cool the exhaust gases in order to avoid damage to the filter media in the baghouse and/or to permit operation of the facility with a higher proportion of RAP and/or RAS.
Notes on Construction
The use of the terms “a”, “an”, “the” and similar terms in the context of describing the invention are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising”, “having”, “including” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The terms “substantially”, “generally” and other words of degree are relative modifiers intended to indicate permissible variation from the characteristic so modified. The use of such terms in describing a physical or functional characteristic of the invention is not intended to limit such characteristic to the absolute value which the term modifies, but rather to provide an approximation of the value of such physical or functional characteristic. All methods described herein can be performed in any suitable order unless otherwise specified herein or clearly indicated by context.
Terms concerning attachments, coupling and the like, such as “connected” and “interconnected”, refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both moveable and rigid attachments or relationships, unless specified herein or clearly indicated by context. The term “operatively connected” is such an attachment, coupling or connection that allows the pertinent structures to operate as intended by virtue of that relationship.
The use of any and all examples or exemplary language (e.g., “such as” and “preferably”) herein is intended merely to better illuminate the invention and the preferred embodiments thereof, and not to place a limitation on the scope of the invention. Nothing in the specification should be construed as indicating any element as essential to the practice of the invention unless so stated with specificity. Several terms are specifically defined herein. These terms are to be given their broadest possible construction consistent with such definitions, as follows:
The terms “recycled asphalt product”, “RAP” and similar terms refer to a comminuted or crushed product containing aggregate materials bound together by asphalt cement. RAP typically comprises crushed or comminuted recycled asphalt concrete.
The terms “recycled asphalt shingles”, “RAS” and similar terms refer to crushed, shredded or comminuted asphalt roofing shingles and/or asphalt cement-containing products other than RAP.
The term “aggregate materials” and similar terms refer to crushed stone and other particulate materials that are used in the production of asphalt concrete, such as, for example, crushed limestone and other types of crushed stone, crushed Portland cement concrete, shredded or comminuted mineral and cellulosic fibers, RAP, RAS, gravel, sand, lime and other particulate additives. The term “virgin aggregate materials” refers to aggregate materials that do not include asphalt cement.
The term “asphalt cement” and similar terms refer to a bituminous material that is used in combination with aggregate materials in the production of asphalt concrete. Asphalt cement acts as the binder for various aggregate materials in the production of asphalt concrete.
The term “asphalt concrete” and similar terms refer to a bituminous paving mixture that is produced, using asphalt cement and/or RAP and/or RAS and any of various aggregate materials, in an asphalt dryer/mixer or other asphalt concrete production plant.
The term “direct dryer”, “direct-fired dryer” and similar terms refer to a dryer or dryer/mixer device having a burner, which device is adapted to move input feed materials to be heated and/or dried along the device through the heated gases generated by the burner in either parallel flow or counter-current flow to an outlet.
The term “indirect dryer” and similar terms refer to a dryer or dryer/mixer device which is adapted to heat and/or dry input feed materials without such materials coming into direct contact with a burner flame or heated gases generated by a burner.
The term “dryer” includes direct dryers and dryer/mixers and indirect dryers and dryer/mixers.
The term “downstream”, as used herein to describe a relative position on or in connection with an asphalt concrete production facility or a component thereof, refers to a relative position in the direction of the movement of material, air or gases through the facility or component thereof.
The term “upstream”, as used herein to describe a relative position on or in connection with an asphalt concrete production facility or a component thereof, refers to a relative position in a direction that is opposite to the direction of the movement of material, air or gases through the facility or component thereof.